1. Field of the Invention
The present invention relates to an exposure apparatus and a device manufacturing method using the exposure apparatus, and, more particularly, to an exposure apparatus and a device manufacturing method using the exposure apparatus for manufacturing ICs, LSIs, CCDs and liquid-crystal panels.
2. Description of the Related Art
A projection aligner (e.g., a projection exposure apparatus) for manufacturing semiconductor devices projects a luminous flux from its illumination system (e.g., an illumination optical system) to a reticle having an electrical circuit pattern formed thereon, and exposes a wafer to the circuit pattern through a projection system (projection optical system). To enhance the resolution, the reticle surface, as a surface to be illuminated, and a wafer surface need to be uniformly illuminated.
The illumination system used in the projection aligner adopts a variety of methods for uniformly illuminating the surface. A step-and-repeat type projection aligner, simply called a stepper, employs an illumination system which is combined with a collimator lens and an optical integrator. The optical integrator is composed of a plurality of micro-lenses arranged with a predetermined pitch. The stepper thus uniformly illuminates the surface.
With the optical integrator employed, the illumination system forms secondary light sources of a number equal to the number of micro-lenses. Luminous fluxes from the secondary light sources illuminate the surface from different directions, thereby assuring a uniform illuminance distribution on the surface.
An illuminance non-uniformity S, as a measure of an illuminance unevenness on the surface to be illuminated, is expressed as Smax, representing a maximum value of the illuminance on the surface and Smin, representing a minimum value of the illuminance on the surface, as follows: EQU S=(Smax-Smin)/(Smax+Smin).
Conventional projection aligners control the illuminance unevenness on the surface to be illuminated to within several percent.
As a high degree of integration is promoted in ULSIs (Ultra Large Scale Integrated Circuits) today, the illuminance uniformity required in the projection of the circuit pattern is as severe as .+-.1%. The conventional art alone cannot fully meet such a rigorous requirement.
To enhance the resolution of the aligner, an off-axis illumination method and a super-high resolution technique employing a phase shift mask have been proposed. In such super-high resolution techniques, a .sigma. value, which is the ratio of NAs (numerical apertures) of the illumination system and the projection system, is reduced by modifying the aperture of the illumination optical system. Alternatively, a secondary light source having an annular shape or a quadruple-pole shape is employed.
In contrast to these illumination methods (illumination modes) employing different apertures, many projection aligners adjust the position of the elements in the illumination system so that the illuminance non-uniformity is minimized in a standard illumination mode A.
When the standard illumination mode A is switched to an illumination mode B, different from the illumination mode A, such as the off-axis illumination method or small a method, the illuminance non-uniformity is not always minimized if the elements in the illumination system remain the same as in the illumination mode A.
A diversity of optical elements is employed in the optical systems in the projection aligner. These optical elements are typically provided with an anti-reflection coating. Since the optical effect of the anti-reflection coating changes as the angle of a light ray varies, peripheral illuminance on the edge of the surface to be illuminated varies when the illumination mode is switched, and illuminance non-uniformity symmetrical with respect to the optical axis takes place. When the illumination mode is switched, illuminance non-uniformity asymmetrical with respect to the optical axis takes place on the surface to be illuminated. This is because the switching of the illumination mode causes variations in the effect of the reflection non-uniformity arising from reflection from mirrors and decentralization of the optical system, and in the effect of flare arising from reflections taking place among the wafer surface, the reticle surface, the projection optical system and the illumination optical system.,
In the conventional projection aligner, concurrently correcting the illuminance non-uniformity symmetrical with respect to the optical axis and the illuminance non-uniformity asymmetrical with respect to the optical axis is difficult when the illumination conditions are switched.